Applicability of partial monomer volumes derived from the regular solution theory

We demonstrate that the regular solution theory is qualitatively successful in describing the composition dependence of partial monomer volumes in a compressible binary mixture of polymers only when the mixture is symmetric (see text for proper definition). For asymmetric mixtures, the partial monomer volumes can be non-monotonic with composition under ordinary conditions of temperature and pressure, which contradicts the prediction of the regular solution theory. The investigation enables us to conclude that the composition dependence of the partial monomer volumes obtained from the regular solution theory can be considered trustworthy only for a low degree of asymmetry.